It is known to include signal rejection filters prior to the RF tuner of a television receiver to inhibit undesired signals from reaching the tuner and therefore interfering with the proper reception of desired signals. For example, highly selective rejection filters or traps have been provided to the tuner of a television to reject FM signals. For the most part, the traps that have been employed have been fixed-tuned, i.e., have employed capacitors and inductors which have fixed values, to reject a specific undesired signal. It is also known to employ both mechanical and electrical switching arrangements to switch the fixed elements of the traps in accordance with the band of desired signal. Such prior pre-tuner trap arrangements have not been adjustable as a function of the channel selected and therefore have been unable to effectively reject undesired variable frequency signals specifically associated with each individual channel.
It is also known to provide tunable traps between the RF input and tuner of a television receiver to attenuate specific variable frequency signals offset in frequency by a predetermined amount from the desired frequency tuned in accordance with the selected channel. For example, U.S. Pat. No. 3,696,302, entitled "UHF-VHF Varactor Tuner Amplifying Band Converter", issued in the name of Gossard on Oct. 3, 1972, discloses a double conversion tuner, i.e., one with two successive heterodyning stages, in which a tunable trap for inhibiting the first local oscillator signal from reaching the RF input is located prior to the tuner in each of a UHF and VHF section. In addition, a tunable trap for inhibiting an RF image signal (i.e., a RF signal having a frequency equal to the sum of the local oscillator frequency and the IF frequency) from reaching the tuner is commonly located prior to the tuner in the VHF section. While such traps are effective to attenuate undesired signals at the trap frequencies, at other frequencies the traps present residual reactances which are uncompensated and therefore may detune other circuits at the desired frequencies.